WASHINGTON -- Pilots of a Southwest Airlines jet took too long after touchdown to use their engines to slow down, causing the plane to slide off a snowy Chicago runway and kill a 6-year-old boy in a passing car, federal safety investigators ruled Tuesday.

While finding the pilots at fault in the 2005 accident, the bulk of the National Transportation Safety Board's (NTSB) report focused on broad failures by the airline and an inadequate system of measuring slick runways that gave pilots a false sense of security as they approached Midway Airport.

"This crew knew they were flying on the edge," board member Debbie Hersman said. "The problem was they didn't really know where the edge was."

Though most large airlines are now more cautious when calculating how to land in snow, many smaller carriers have not adopted recommended improvements, the safety board said.

The Federal Aviation Administration did not identify the airlines.

The NTSB is investigating two less serious accidents earlier this year in which regional jets skidded off snowy runways under similar circumstances.

Investigators issued several recommendations calling on the FAA and airlines to tighten up how flight crews calculate whether it is safe to land. It also called on the FAA to develop a system that uses jets' onboard computers to calculate a runway's slickness and transmit that data to other pilots.

The accident on Dec. 8, 2005, off the northwestern boundary of Midway, killed Joshua Woods, 6, who was riding in a car with his family as snow pelted Chicago.

Because conditions were slick and Midway's runways are shorter than at most other airports, the pilots on Southwest Flight 1248 decided to use a new braking system that would help the jet stop, the NTSB said. But Capt. Bruce Sutherland told investigators that he got distracted by the brakes, which slow the jet automatically. For 18 critical seconds, the pilots neglected to use thrust reversers, which also help jets slow down by redirecting engine exhaust forward.

Investigators concluded that the pilots could have stopped the Boeing 737-700 on the runway if they had used thrust reversers at maximum power until the jet stopped, an aggressive maneuver typically used only in an emergency.

As they approached the airport, the pilots received numerous reports that indicated the runway conditions ranged from "good" to "fair" to "poor."

Southwest's policy was that pilots use the worst report as the basis for calculating whether they could stop, but the pilots said they were not aware of this rule. They based their landing on an assumption that conditions were fair. Under Southwest's rules, they should have diverted to another airport if the runway was in poor condition.

The investigation showed that misunderstandings and mistakes stacked up to cause the accident.

For example, subjective reports about a runway's condition, relayed from pilot to pilot over the radio, are used by airlines to determine whether it's safe to land. The NTSB found the reports were not accurate the night of the Midway accident. Computer calculations after the crash showed that the actual conditions were worse than poor.

Also, a computer aboard the jet told pilots they had just enough runway on which to stop, but the results were misleading. It did not take into account the full tail wind at the time. It also assumed the plane could stop under the conditions in a shorter distance than Boeing had recommended.